Reflective polarizers containing polymer fibers

ABSTRACT

A polarizer is formed with an arrangement of polymer fibers substantially parallel within a polymer matrix. The polymer fibers are formed of at least first and second polymer materials. At least one of the polymer matrix and the first and second polymer materials is birefringent, and provides a birefringent interface with the adjacent material. Light is reflected and/or scattered at the birefringent interfaces with sensitivity to the polarization of the light. In some embodiments, the polymer fibers are formed as composite fibers, having a plurality of scattering polymer fibers disposed within a filler to form the composite fiber. In other embodiments, the polymer fiber is a multilayered polymer fiber. The polymer fibers may be arranged within the polymer matrix as part of a fiber weave.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/068,157, filed Feb. 28, 2005, now U.S. Pat. No. ______. Thisapplication is also related to co-owned U.S. patent application Ser.Nos. 11/068,158, titled “COMPOSITE POLYMER FIBERS”, 11/068,159, titled“COMPOSITE POLYMERIC OPTICAL FILMS WITH CO-CONTINUOUS PHASES”,11/068,590 titled “OPTICAL ELEMENTS CONTAINING A POLYMER FIBER WEAVE”,11/067,848 titled “POLYMER PHOTONIC CRYSTAL FIBERS”, and 11/068,313,titled “POLYMERIC PHOTONIC CRYSTALS WITH CO-CONTINUOUS PHASES”, allfiled on Feb. 218, 2005 and all of which are incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to polymer optical film and more particularly toreflective polarizing polymer optical film that contains polymer fibers.

BACKGROUND

Unpolarized light waves vibrate in a large number of planes about theaxis of a light beam. If the waves vibrate in one plane only, the lightis said to be plane polarized. Several useful optical systems can beimplemented using polarized light. For example, electro-optical devicessuch as liquid crystal display screens are illuminated with polarizedlight and use crossed polarizers in conjunction with an addressableliquid crystal interlayer to provide the basis for displaying imageformation. In the field of photography, polarizing filters have beenused to reduce the glare and the brightness of specular reflection.Polarizing filters, circular polarizers or other optical components havealso been used for glare reduction in display device screens.

Several different kinds of polarizing film are available for polarizingunpolarized light. Absorbing (dichroic) polarizers have, as an inclusionphase, polarization-dependent absorbing species, often aniodine-containing chain, that are aligned within a polymer matrix. Sucha film absorbs light polarized with its electric field vector alignedparallel to absorbing species and transmits light polarizedperpendicular to the absorbing species. The optical properties of such afilm are typically specular, with very little diffuse transmissionthrough the film or diffuse reflection from the film surfaces.

Another type of polarizing film is a reflective polarizer, whichseparates light in different polarization states by transmitting lightin one state and reflecting light in the other state. One type ofreflective polarizer is a multilayer optical film (MOF), which is a filmformed of a stack of many layers of alternating polymer materials. Oneof the materials is optically isotropic while the other is birefringent,with one of its refractive indices matched to that of the isotropicmaterial. The layer thicknesses may be varied throughout the stack so asto be quarter wave layers over a wide range of wavelengths, for exampleover the visible region. Light incident in one polarization stateexperiences the matched refractive indices and is substantiallyspecularly transmitted through the polarizer. Light incident in theother polarization state, however, experiences multiple coherent orincoherent reflections at the interfaces between the different layersand is reflected by the polarizer. Since the alternating polymer layersare substantially planar, the reflected light is mostly specularlyreflected.

Another type of reflective polarizing film is constructed frominclusions dispersed within a continuous phase matrix. The inclusionsare small relative to the width and height of the film. Thecharacteristics of these inclusions can be manipulated to provide arange of reflective and transmissive properties to the film. Theinclusions constitute a disperse polymer phase within the continuousphase matrix. The inclusion size and alignment can be altered bystretching the film. Either the continuous phase or the disperse phaseis birefringent, with one of the refractive indices of the birefringentmaterial matching to the refractive index of the other phase, which isoptically isotropic. Selection of the materials for the continuous anddisperse phases, along with the degree of stretching, can affect thedegree of birefringent refractive index mismatch between the dispersephase and the continuous phase. Other characteristics that can beadjusted include the inclusion size with respect to wavelength withinthe film, the inclusion shape and the inclusion volumetric fill factor.In such systems, light polarized to experience the refractive indexmismatch between the disperse and continuous phases is diffuselyreflected, whereas the orthogonally polarized light is specularlytransmitted.

SUMMARY OF THE INVENTION

One particular embodiment of the invention is directed to an opticalbody that comprises a polymer matrix and a plurality of polymer fibersdisposed within the polymer matrix. At least one of the polymer fiberscomprises multiple birefringent interfaces for reflecting light. Thebirefringent interfaces are formed between a first polymer material anda second polymer material, at least one of the first and second polymermaterials being birefringent. The birefringent interfaces are internalto the polymer fiber and are elongated along an axis of the polymerfiber. The polymer fibers are oriented substantially parallel to a firstaxis. A refractive index difference at the birefringent interfaces forlight polarized parallel to the first axis is different from arefractive index difference at the birefringent interfaces for lightpolarized parallel to a second axis, the second axis being orthogonal tothe first axis.

Another embodiment of the invention is directed to an optical body thatcomprises an optical material having a first refractive index, n_(x),along a first axis and a second refractive index, n_(y), along a secondaxis perpendicular to the first axis. Elongated polymer fibers aredisposed within the optical material parallel to the first axis. Thepolymer fibers comprise a first birefringent material elongated alongthe first axis and having a refractive index, n_(1x), along the firstaxis and a refractive index n_(1y) along the second axis, where n_(1x)is different from n_(1y). The elongated polymer fibers comprise a secondmaterial elongated along the first axis and having a refractive indexn_(2x) along the first axis and a refractive index n_(2y) along thesecond axis. The optical body scatters incident light polarized alongthe first axis differently than incident light polarized along thesecond axis.

Another embodiment of the invention is directed to an optical body thatcomprises a polymer matrix and a plurality of polymer fibers disposedwithin the polymer matrix. At least one of the polymer fibers comprisesmultiple, elongated birefringent scattering means for scatteringincident light, the scattering means being provided internally withinthe polymer fibers.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIGS. 1A and 1B schematically illustrate polarizers that demonstratespecular reflection and diffuse reflection respectively;

FIG. 2 schematically illustrates a cut-away view of an embodiment of apolarizer according to principles of the present invention;

FIGS. 3A-3D schematically illustrate cross-sectional views throughembodiments of optical elements according to principles of the presentinvention;

FIG. 3E schematically illustrates an embodiment of an optical element inwhich not all the polymer fibers are parallel, according to principlesof the present invention;

FIGS. 3F-3H schematically illustrate cross-sectional views throughembodiments of optical elements according to principles of the presentinvention;

FIGS. 3I-3M schematically illustrate cross-sectional views throughembodiments of optical elements having structured surfaces according toprinciples of the present invention;

FIGS. 4A-4C schematically illustrate cross-sectional views throughmultilayered fibers according to principles of the present invention;

FIGS. 4D-4G schematically illustrate cross-sectional views throughpolarizers using multilayered fibers according to principles of thepresent invention;

FIGS. 5A-5K schematically illustrate cross-sectional views throughembodiments of composite fibers according to principles of the presentinvention;

FIGS. 6A-6I schematically illustrate cross-sectional views throughembodiments of composite fibers according to principles of the presentinvention;

FIG. 7 presents a graph showing light scattering efficiency as afunction of scattering fiber radius;

FIG. 8A schematically illustrates an embodiment of a disperse phasebirefringent polymer fiber according to principles of the presentinvention;

FIG. 8B schematically illustrates an embodiment of a disperse phasebirefringent composite polymer fiber according to principles of thepresent invention;

FIG. 9 schematically illustrates a polymer fiber yarn for use in apolarizer according to principles of the present invention;

FIGS. 10A-10D schematically illustrate steps in an embodiment of amethod of fabricating a polymer-fiber optical element according toprinciples of the present invention;

FIG. 11 schematically illustrates a fiber tow used in an embodiment of amethod of fabricating a polymer-fiber optical element according toprinciples of the present invention;

FIG. 12 schematically illustrates a fiber weave used in an embodiment ofa method of fabricating a polymer-fiber optical element according toprinciples of the present invention;

FIGS. 13A and 13B schematically illustrate cross-sectional views ofembodiments of a polymer fiber weave as may be used in a polymer-fiberoptical element according to principles of the present invention; and

FIG. 14 shows a photograph illustrating a cross-section through ascattering fiber that may be used in a polarizer according to principlesof the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to optical systems and is moreparticularly applicable to polarized optical systems.

As used herein, the terms “specular reflection” and “specularreflectance” refer to the reflectance of light rays from a body wherethe angle of reflection is substantially equal to the angle ofincidence, where the angles are measured relative to a normal to thebody's surface. In other words, when the light is incident on the bodywith a particular angular distribution, the reflected light hassubstantially the same angular distribution. The terms “diffusereflection” or “diffuse reflectance” refer to the reflection of rayswhere the angle of some of the reflected light is not equal to the angleof incidence. Consequently, when light is incident on the body with aparticular angular distribution, the angular distribution of thereflected light is different from that of the incident light. The terms“total reflectance” or “total reflection” refer to the combinedreflectance of all light, specular and diffuse.

Similarly, the terms “specular transmission” and “speculartransmittance” are used herein in reference to the transmission of lightthrough a body where the angular distribution of the transmitted lightis substantially the same as that of the incident light. The terms“diffuse transmission” and “diffuse transmittance” are used to describethe transmission of light through a body, where the transmitted lighthas an angular distribution that is different from the angulardistribution of the incident light. The terms “total transmission” or“total transmittance” refer to the combined transmission of all light,specular and diffuse.

A reflective polarizer 100 in the form of a film is schematicallyillustrated in FIGS. 1A and 1B. In the convention adopted herein, thethickness direction of the film is taken as the z-axis, and x-y plane isparallel to the plane of the film. When unpolarized light 102 isincident on the polarizer 100, the light 104 polarized parallel to thetransmission axis of the polarizer 100 is transmitted, while the light106 polarized parallel to the reflection axis of the polarizer 100 isreflected. The angular distribution of the reflected light is dependenton various characteristics of the polarizer 100. For example, in someexemplary embodiments the light 106 may be specularly reflected, as isschematically illustrated in FIG. 1A, while in other embodiments thelight 106 may be diffusely reflected, as is schematically illustrated inFIG. 1B. In other embodiments, the reflected light may include bothspecular and diffuse components. In the illustrated embodiment, thetransmission axis of the polarizer is parallel to the x-axis and thereflection axis of the polarizer 100 is parallel to the y-axis. In otherembodiments, these may be reversed. The transmitted light 104 may bespecularly transmitted, diffusely transmitted, or may be transmittedwith a combination of specular and diffuse components.

A cut-away view through a reflective polarizer body according to anexemplary embodiment of the present invention is schematically presentedin FIG. 2. The optical body 200 comprises a polymer matrix 202, alsoreferred to as a continuous phase. The polymer matrix may be opticallyisotropic or optically birefringent. For example, the polymer matrix maybe uniaxially or biaxially birefringent, meaning that the refractiveindex of the polymer may be different along one direction and similar intwo orthogonal directions (uniaxial) or different in all threeorthogonal directions (biaxial).

Polymer fibers 204 are disposed within the matrix 202. The polymerfibers 204 comprise at least two materials. In some exemplaryembodiments, one of the materials is birefringent while the othermaterial, or materials, is/are isotropic. In other embodiments, two ormore of the materials forming the fiber are birefringent. Also, in someother embodiments, the materials forming the fiber may be isotropic. Inother embodiments, both isotropic and birefringent polymer fibers 204may be disposed within the matrix 202.

The polymer fibers 204 may be organized within the matrix 202 as singlefibers, as illustrated, or in many other arrangements. Some exemplaryarrangements include yarns, a tow (of fibers or yarns) arranged in onedirection within the polymer matrix, a weave, a non-woven, choppedfiber, a chopped fiber mat (with random or ordered formats), orcombinations of these formats. The chopped fiber mat or nonwoven may bestretched, stressed, or oriented to provide some alignment of the fiberswithin the nonwoven or chopped fiber mat, rather than having a randomarrangement of fibers.

The refractive indices in the x-, y-, and z-directions for the firstfiber material may be referred to as n_(1x), n_(1y) and n_(1z), and therefractive indices in the x-, y-, and z-directions for the second fibermaterial may be referred to as n_(2x), n_(2y) and n_(2z). Where thematerial is isotropic, the x-, y-, and z-refractive indices are allsubstantially matched. Where the first fiber material is birefringent,at least one of the x-, y- and z-refractive indices is different fromthe others.

There are multiple interfaces within each fiber 204 between the firstfiber material and the second fiber material. Where at least one of thematerials forming the interface is birefringent, the interface isreferred to as a birefringent interface. For example, if the twomaterials present their x- and y-refractive indices at the interface,and n_(1x)≠n_(1y), i.e., the first material is birefringent, then theinterface is birefringent. Different exemplary embodiments of thepolymer fibers containing birefringent interfaces are discussed below.

The fibers 204 are disposed generally parallel to an axis, illustratedas the x-axis in the figure. The refractive index difference at thebirefringent interfaces within the fibers 204 for light polarizedparallel to the x-axis, n_(1x)−n_(2x), may be different from therefractive index difference for light polarized parallel to the y-axis,n_(1y)−n_(2y). Thus, for one polarization state, the refractive indexdifference at the birefringent interfaces in the fibers 204 may berelatively small. In some exemplary cases, the refractive indexdifference may be less than 0.05. This condition is considered to besubstantially index-matched. This refractive index difference may beless than 0.03, less than 0.02, or less than 0.01. If this polarizationdirection is parallel to the x-axis, then x-polarized light passesthrough the body 200 with little or no reflection. In other words,x-polarized light is highly transmitted through the body 200.

The refractive index difference at the birefringent interfaces in thefibers may be relatively high for light in the orthogonal polarizationstate. In some exemplary examples, the refractive index difference maybe at least 0.05, and may be greater, for example 0.1, or 0.15 or may be0.2. If this polarization direction is parallel to the y-axis, theny-polarized light is reflected at the birefringent interfaces. Thus,y-polarized light is reflected by the body 200. If the birefringentinterfaces within the fibers 204 are substantially parallel to eachother, then the reflection may be essentially specular. If, on the otherhand, the birefringent interfaces within the fibers 204 are notsubstantially parallel to each other, then the reflection may besubstantially diffuse. Some of the birefringent interfaces may beparallel, and other interfaces may be non-parallel, which may lead tothe reflected light containing both specular and diffuse components.Also, a birefringent interface may be curved, or relatively small, inother words within an order of magnitude of the wavelength of theincident light, which may lead to diffuse scattering.

While the exemplary embodiment just described is directed to indexmatching in the x-direction, with a relatively large index difference inthe y-direction, other exemplary embodiments include index matching inthe y-direction, with a relatively large index difference in thex-direction.

The polymer matrix 202 may be substantially optically isotropic, forexample having a birefringence, n_(3x)−n_(3y), of less than about 0.05,and preferably less than 0.01, where the refractive indices in thematrix for the x- and y-directions are n_(3x) and n_(3y) respectively.In other embodiments, the polymer matrix 202 may be birefringent.Consequently, in some embodiments, the refractive index differencebetween the polymer matrix and the fiber materials may be different indifferent directions. For example, the x-refractive index difference,n_(1x)−n_(3x), may be different from the y-refractive index difference,n_(1y)−n_(3y). In some embodiments, one of these refractive indexdifferences may be at least twice as large as the other refractive indexdifference.

In some embodiments, the refractive index difference, the extent andshape of the birefringent interfaces, and the relative positions of thebirefringent interfaces may result in diffuse scattering of one of theincident polarizations more than the other polarization. Such scatteringmay be primarily back-scattering (diffuse reflection) forward-scattering(diffuse transmission) or a combination of both back- andforward-scattering.

Suitable materials for use in the polymer matrix and/or in the fibersinclude thermoplastic and thermosetting polymers that are transparentover the desired range of light wavelengths. In some embodiments, it maybe particularly useful that the polymers be non-soluble in water.Further, suitable polymer materials may be amorphous orsemi-crystalline, and may include homopolymer, copolymer or blendsthereof. Example polymer materials include, but are not limited to,poly(carbonate) (PC); syndiotactic and isotactic poly(styrene) (PS);C1-C8 alkyl styrenes; alkyl, aromatic, and aliphatic ring-containing(meth)acrylates, including poly(methylmethacrylate) (PMMA) and PMMAcopolymers; ethoxylated and propoxylated (meth)acrylates;multifunctional (meth)acrylates; acrylated epoxies; epoxies; and otherethylenically unsaturated materials; cyclic olefins and cyclic olefiniccopolymers; acrylonitrile butadiene styrene (ABS); styrene acrylonitrilecopolymers (SAN); epoxies; poly(vinylcyclohexane);PMMA/poly(vinylfluoride) blends; poly(phenylene oxide) alloys; styrenicblock copolymers; polyimide; polysulfone; poly(vinyl chloride);poly(dimethyl siloxane) (PDMS); polyurethanes; unsaturated polyesters;poly(ethylene), including low birefringence polyethylene;poly(propylene) (PP); poly(alkane terephthalates), such as poly(ethyleneterephthalate) (PET); poly(alkane napthalates), such as poly(ethylenenaphthalate) (PEN); polyamide; ionomers; vinyl acetate/polyethylenecopolymers; cellulose acetate; cellulose acetate butyrate;fluoropolymers; poly(styrene)-poly(ethylene) copolymers; PET and PENcopolymers, including polyolefinic PET and PEN; andpoly(carbonate)/aliphatic PET blends. The term (meth)acrylate is definedas being either the corresponding methacrylate or acrylate compounds.With the exception of syndiotactic PS, these polymers may be used in anoptically isotropic form.

Several of these polymers may become birefringent when oriented. Inparticular, PET, PEN, and copolymers thereof, and liquid crystalpolymers, manifest relatively large values of birefringence whenoriented. Polymers may be oriented using different methods, includingextrusion and stretching. Stretching is a particularly useful method fororienting a polymer, because it permits a high degree of orientation andmay be controlled by a number of easily controllable externalparameters, such as temperature and stretch ratio. The refractiveindices for a number of exemplary polymers, oriented and unoriented, areprovided in Table I below.

TABLE I Typical Refractive Index Values for Some Polymer MaterialsResin/Blend S.R. T (° C.) n_(x) n_(y) n_(z) PEN 1 — 1.64 PEN 6 150 1.881.57 1.57 PET 1 — 1.57 PET 6 100 1.69 1.54 1.54 CoPEN 1 — 1.57 CoPEN 6135 1.82 1.56 1.56 PMMA 1 — 1.49 PC, CoPET blend 1 — 1.56 THV 1 — 1.34PETG 1 — 1.56 SAN 1 — 1.56 PCTG 1 — 1.55 PS, PMMA copolymer 1 —1.55-1.58 PP 1 — 1.52 Syndiotactic PS 6 130 1.57 1.61 1.61

PCTG and PETG (a glycol-modified polyethylene terephthalate) are typesof copolyesters available from, for example, Eastman Chemical Co.,Kingsport, Tenn., under the Eastar™ brand name. THV is a polymer oftetrafluoroethylene, hexafluoropropylene and vinylidene fluoride,available from 3M Company, St. Paul, Minn., under the brand nameDyneon™. The PS/PMMA copolymer is an example of a copolymer whoserefractive index may be “tuned” by changing the ratio of the constituentmonomers in the copolymer to achieve a desired value of refractiveindex. The column labeled “S.R.” contains the stretch ratio. A stretchratio of 1 means that the material is unstretched and unoriented. Astretch ratio of 6 means that sample was stretched to six times itoriginal length. If stretched under the correct temperature conditions,the polymeric molecules are oriented and the material becomesbirefringent. It is possible, however, to stretch the material withoutorienting the molecules. The column labeled “T” indicates thetemperature at which the sample was stretched. The stretched sampleswere stretched as sheets. The columns labeled n_(x), n_(y) and n_(z)refer to the refractive indices of the material. Where no value islisted in the table for n_(y) and n_(z), the values of n_(y) and n_(z)are the same as for n_(x).

The behavior of the refractive index under stretching a fiber isexpected to give results similar to, but not necessarily the same as,those for stretching a sheet. Polymer fibers may be stretched to anydesired value that produces desired values of refractive index. Forexample, some polymer fibers may be stretched to produce a stretch ratioof at least 3, and maybe at least 6. In some embodiments, polymer fibersmay be stretched even more, for example to a stretch ratio of up to 20,or even more.

A suitable temperature for stretching to achieve birefringence isapproximately 80% of the polymer melting point, expressed in Kelvins.Birefringence may also be induced by stresses induced by flow of thepolymer melt experienced during extrusion and film formation processes.Birefringence may also be developed by alignment with adjacent surfacessuch as fibers in the film article. Birefringence may either be positiveor negative. Positive birefringence is defined as when the direction ofthe electric field axis for linearly polarized light experiences thehighest refractive index when it is parallel to the polymer'sorientation or aligning surface. Negative birefringence is defined aswhen the direction of the electric field axis for linearly polarizedlight experiences the lowest refractive index when it is parallel to thepolymer's orientation or aligning surface. Examples of positivelybirefringent polymers include PEN and PET. An example of a negativelybirefringent polymer includes syndiotactic polystyrene.

The matrix 202 and/or the polymer fibers 204 may be provided withvarious additives to provide desired properties to the optical body 200.For example, the additives may include one or more of the following: ananti-weathering agent, UV absorbers, a hindered amine light stabilizer,an antioxidant, a dispersant, a lubricant, an anti-static agent, apigment or dye, a nucleating agent, a flame retardant and a blowingagent. Other additives may be provided for altering the refractive indexof the polymer or increasing the strength of the material. Suchadditives may include, for example, organic additives such as polymericbeads or particles and polymeric nanoparticles, or inorganic additives,such as, ceramic or metal-oxide nanoparticles, or milled, powered, bead,flake or particulate glass, ceramic or glass-ceramic. The surface ofthese additives may be provided with a binding agent for binding to thepolymer. For example, a silane coupling agent may be used with a glassadditive to bind the glass additive to the polymer.

In some embodiments, it may be preferable that the matrix 202 or acomponent of the polymer fibers 204 be non-soluble, or at leastresistant to solvents. Examples of suitable materials that are solventresistant include polypropylene, PET and PEN. In other embodiments itmay be preferable that the matrix 202 or component of the polymer fibers204 is soluble in an organic solvent. For example, a matrix 202 or fibercomponent formed of polystyrene is soluble in an organic solvent such asacetone. In other embodiments, it may be preferable that the matrix iswater soluble. For example, a matrix 202 or fiber component formed ofpolyvinyl acetate is soluble in water.

The refractive index of the materials in some embodiments of opticalelement may vary along the length of the fiber, in the x-direction. Forexample, the element may not be subject to uniform stretching, but maybe stretched to a greater degree in some regions than in others.Consequently, the degree of orientation of the orientable materials isnot uniform along the element, and so the birefringence may varyspatially along the element.

Furthermore, the incorporation of fibers within the matrix may improvethe mechanical properties of the optical element. In particular, somepolymeric materials, such as polyester, are stronger in the form of afiber than in the form of a film, and so an optical element containingfibers may be stronger than one of similar dimensions that contains nofibers.

The polymer fibers 204 may be straight, but need not be straight, forexample the polymer fibers 204 may be kinked, spiraled or crimped.

The polymer fibers 204 may be arranged within the matrix 202 in manydifferent ways. For example, the fibers 204 may be positioned randomlyacross the cross-sectional area of the matrix 202: in FIG. 2, theposition of different fibers 204 in the y-z plane is random. Othercross-sectional arrangements may be used. For example, in the exemplaryembodiment schematically illustrated in FIG. 3A, which shows across-section through an optical element 300, the fibers 304 arearranged in a one-dimensional array within the matrix 302, with regularspacing between adjacent fibers 304. In some variations of thisembodiment, the spacing between adjacent fibers 304 need not be the samefor all fibers 304. The optical element 300 may be a polarizer.

In another exemplary embodiment, schematically illustrated in FIG. 3B asa cross-section through an optical element 310, the fibers 314 arearranged in a regular two-dimensional array within the matrix 312. Inthe illustrated embodiment, the separation distance between adjacentfibers 314 in the y-direction, h_(y), is the same as the separationdistance between adjacent fibers in the z-direction, h_(z). This neednot be the case, and the separation distance in the z-direction, h_(z),may be different from the separation distance in the y-direction, h_(y),as is schematically illustrated in FIG. 3C. The optical element 310 maybe a polarizer.

In another embodiment, the fibers 314 may be offset between adjacentrows, for example as is schematically illustrated in FIG. 3D, creatingan hexagonally-packed fiber pattern. Other regular patterns of fibers314 may be employed, or irregular patterns of fibers 314 may beemployed.

While the fibers 314 may all be substantially parallel to the x-axis,this need not be the case, and some fibers 314 may lie with greater orsmaller angles to the x-axis. For example, in the example opticalelement 310 illustrated in FIG. 3D, and further illustrated in FIG. 3E,the first row 316 a of fibers 314 may be oriented so as that the fibers314 lie parallel to each other in a plane parallel to the y-z plane, butat a first angle, θ1, relative to the x-axis. The fibers 314 in thesecond row 316 b may also lie parallel to each other within a planeparallel to the y-z plane, but at a second angle, θ2, to the x-axis, notnecessarily equal to the first angle. Also, the fibers 314 in the thirdrow 316 c may lie parallel to teach other in a plane parallel to the y-zaxis, but at a third angle, θ3, relative to the x-axis. The third anglemay or may not be equal to either the first or second angles. In theillustrated embodiment, the value of θ3 is equal to zero, and the fibers314 in the third row 316 c are parallel to the x-axis. The differentvalues of θ1, θ2, and θ3 may, however, reach up to 90°.

Such an arrangement can be particularly useful where the fibers in onerow are effective for light in a first wavelength band and the fibers inanother row are effective for light in a second wavelength banddifferent from the first wavelength band. Consider the illustrativeexample where the fibers 314 in the first row 316 a are effective atreflectively polarizing light in a red bandwidth and the fibers 314 inthe second row 316 b are effective at reflectively polarizing light in ablue bandwidth. Therefore, if the optical element 310 were to beilluminated with a mixture of red and blue light, the first row 316 a offibers 314 would pass all the blue light while transmitting red lightpolarized at the angle θ1. The second row 316 a of fibers 314 wouldtransmit the red light polarized at the angle θ1 while also transmittingblue light polarized parallel to the angle θ2. If the angles θ1 and θ2were separated by 90°, the element 310 would transmit red light in onepolarization state and blue light in the orthogonal polarization state.Likewise, the reflected blue light is polarized orthogonally to thereflected red light. It will be appreciated that different numbers ofrows of fibers 314 may be aligned at each angle, and be used for eachcolor band.

In some embodiments, the density of the fibers 314 may be constantwithin the optical element 300, 310, or may vary within the opticalelement 300, 310. For example, the density of fibers 314 may decreasefrom one side of the optical element 300, 310, or may vary in some othermanner. To illustrate this further, FIG. 3F schematically illustrates anembodiment of an optical element 310 in which the density of fibers 314varies across the element 310. In particular the spacing betweenadjacent fibers in the y-direction is not constant for all positions ofy across the element 310. FIG. 3G schematically illustrates an opticalelement 310 in which the density of the fibers 314 varies through theelement 310. In particular, the spacing between adjacent fibers in thez-direction is not constant for all positions of z through the element310. Other variations are possible, for example the spacing betweennearest neighbor fibers may vary in both the y-direction and in thez-direction.

Another embodiment is schematically illustrated in FIG. 3H, in which anoptical element 320 has polymer fibers 324 embedded within a matrix 322.In this particular embodiment, the center-to-center spacing betweenadjacent fibers 324 is reduced in one region, at the center of thefigure, relative to neighboring regions on either side. Consequently,the fill factor, i.e., the fraction of the cross-sectional area of theelement 320 taken up by the fibers 324, is increased in that region.Such a variation in the fill factor may be useful, for example, toimprove the uniformity of light transmitted through the element 320 froma light source 326. This may be important, for example, where theelement 320 is included in a direct view screen lit by discrete lightsources: in such devices it is important to present the viewer with animage of uniform illumination. When a light source is placed behind auniform diffuser, the brightness of the light transmitted through thediffuser is highest above the light source. The variation in fill factorillustrated in FIG. 3H may be used to increase the amount of diffusiondirectly above the light source 326, thus reducing the non-uniformity inthe intensity of the transmitted light.

The optical element of the present disclosure may have flat surfaces,for example the flat surfaces parallel to the x-y plane as shown inFIGS. 1A and 1B. The element may also include one or more surfaces thatare structured to provide desired optical effects for light transmittedthrough, or reflected by, the polarizer. For example, in one exemplaryembodiment schematically illustrated in FIG. 3I, the optical element330, which may be a polarizer, is formed with a matrix 332 containing anumber of polymer fibers 334, and may have one or more curved surfaces336. The curved surfaces 336 provide optical power, focusing ordefocusing, to light transmitted through the surface 336. In theillustrated embodiment, rays 338 represent examples of light rays,polarized parallel to the transmission axis of the optical element 330,that are focused by the curved refracting surface 336. In otherexemplary embodiments, the input surface of the element 330, throughwhich light enters the element 330, may be curved, or there may be othersurface structure. Furthermore, there may be surface structure on theoutput surface, through which transmitted light exits the opticalelement 330. An example of surface structure includes constructions suchas a Fresnel lens structure. Such a structure is also considered toprovide optical power to light passing through the structured surface.

The structured surface of either or both the input and output surfaces,may also include rectilinear regions in addition to, or instead of,curved regions. For example, in another exemplary embodiment,schematically illustrated in FIG. 3J, the optical element 340, formedwith a matrix 342 containing polymer fibers 344, may be provided with aprismatically structured surface 346, referred to as a brightnessenhancing surface. A brightness enhancing surface is commonly used, forexample in backlit liquid crystal displays, to reduce the cone angle ofthe light illuminating the display panel, and thus increase the on-axisbrightness for the viewer. The figure shows an example of two light rays348 and 349 that are non-perpendicularly incident on the element 340.Light ray 348 is in the polarization state that is transmitted by theelement 340, and is also diverted towards the z-axis by the structuredsurface 346. Light ray 349 is in the polarization state that isdiffusely reflected by the element 340. The brightness enhancing surfacemay be arranged so that the prism structures are parallel to the fibers344, which is also parallel to the x-axis, as illustrated. In otherembodiments, the prism structures may lie at some other angle relativeto the direction of the fibers. For example, the ribs may lie parallelto the y-axis, perpendicular to the fibers, or at some angle between thex-axis and the y-axis.

Structured surfaces may be formed on the matrix using any suitablemethod. For example, the matrix may be cured while its surface is incontact with the surface of a tool, such as a microreplication tool,whose tool surface produces the desired shape on the surface of thepolymer matrix.

The polymer fibers may be present across different regions of theoptical element. In FIG. 3J, the polymer fibers 344 are not located inthe structure 347 formed by the structured surface 346, but are locatedonly in the main body 341 of the element 340. In other embodiments, thepolymer fibers 344 may be distributed differently. For example, in theoptical element 350, schematically illustrated in FIG. 3K, the polymerfibers 344 are located within both the main body 341 of the element 350,and also in the structure 347 formed by the structured surface 346. Inanother example, schematically illustrated in FIG. 3L, the polymerfibers 344 are located only in the structure 347 of the element 360 andnot in the main body 341 of the element 360.

Another exemplary embodiment of the invention is schematicallyillustrated in FIG. 3M, in which the element 370 has polymer fibers 374in a matrix 372. In this particular embodiment, some of the fibers 374 aare not completely embedded within the matrix 372, but penetrate thesurface 376 of the matrix 372.

In some exemplary embodiments, the polymer fibers disposed within thepolarizers contain volumes of different polymer materials, including atleast a birefringent material and another material, for example asubstantially non-birefringent material. These different materials maybe arranged in many different ways, for example in regular alternatinglayers, or as fine fibers of one material disposed within a “pool” ofthe other material. Several different exemplary embodiments of polymerfibers containing multiple internal birefringent interfaces arediscussed below. The matrix material may have less birefringence thanthe birefringent material of the fiber, may have no birefringence, ormay be oppositely birefringent. For example, if the birefringentmaterial in the fiber has n_(x)>n_(y), then the matrix material may haven_(y)>n_(x).

In a preferred exemplary embodiment, the birefringent material is of atype that undergoes a change in refractive index upon orientation.Consequently, as the fiber is oriented, refractive index matches ormismatches are produced along the direction of orientation. By carefulmanipulation of orientation parameters and other processing conditions,the positive or negative birefringence of the birefringent material canbe used to induce diffuse reflection or transmission of one or bothpolarizations of light along a given axis. The relative ratio betweentransmission and diffuse reflection is dependent on a number of factorssuch as, but not limited to, the concentration of the birefringentinterfaces in the fiber, the dimension of the fiber, the square of thedifference in the index of refraction at the birefringent interfaces,the size and geometry of the birefringent interfaces, and the wavelengthor wavelength range of the incident radiation.

The magnitude of the index match or mismatch along a particular axisaffects the degree of scattering of light polarized along that axis. Ingeneral, the scattering power varies as the square of the indexmismatch. Thus, the larger the mismatch in refractive index along aparticular axis, the stronger the scattering of light polarized alongthat axis. Conversely, when the mismatch along a particular axis issmall, light polarized along that axis is scattered to a lesser extentand the transmission through the volume of the body becomes increasinglyspecular.

If the index of refraction of the non-birefringent material matches thatof the birefringent material along some axis, then incident lightpolarized with electric fields parallel to this axis will pass throughthe fiber unscattered regardless of the size, shape, and density of theportions of birefringent material. In addition, if the refractive indexalong that axis is also substantially matched to that of the polymermatrix of the polarizer body, then the light passes through the bodysubstantially unscattered. For purposes of this disclosure, substantialmatching between two refractive indices occurs when the differencebetween the indices is less than at most 0.05, and preferably less than0.03, 0.02 or 0.01.

If the indices between the birefringent material and non-birefringentmaterial are not matched along some axis, then the fiber scatters orreflects light polarized along this axis. The strength of the scatteringis determined, at least in part, by the magnitude of the index mismatchfor scatterers having a given cross-sectional area with dimensionslarger than approximately λ/30, where λ is the wavelength of theincident light in the polarizer. The exact size, shape and alignment ofa mismatched interface play a role in determining how much light will bescattered or reflected into various directions from that interface. Ifthe density and thickness of the scattering layer is sufficient,according to multiple scattering theory, incident light will be eitherreflected or absorbed, but not transmitted, regardless of the details ofthe scatterer size and shape.

Prior to use in the polarizer, the fibers are preferably processed bystretching and allowing some dimensional relaxation in the cross stretchin-plane direction, so that the index of refraction difference betweenthe birefringent material and the non-birefringent materials arerelatively large along a first axis and small along the other twoorthogonal axes. This results in a large optical anisotropy forelectromagnetic radiation of different polarizations.

Some of the polarizers within the scope of the present invention areelliptically diffusing polarizers. In general, elliptically diffusingpolarizers use fibers having a difference in index of refraction betweenthe birefringent and non-birefringent materials along both the stretchand non-stretch directions, and may diffusely transmit or reflect lightof one polarization. The birefringent material in the fiber may alsoform birefringent interfaces with the polymer matrix material, in whichcase these interfaces may also include an index mismatch for both thestretch and cross-stretch directions.

The ratio of forward-scattering to back-scattering is dependent on thedifference in refractive index between the birefringent andnon-birefringent materials, the concentration of the birefringentinterfaces, the size and shape of the birefringent interfaces, and theoverall thickness of the fiber. In general, elliptical diffusers have arelatively small difference in index of refraction between thebirefringent and non-birefringent materials.

The materials selected for use in the fibers in accordance with thepresent invention, and the degree of orientation of these materials, arepreferably chosen so that the birefringent and non-birefringentmaterials in the finished fiber have at least one axis for which theassociated indices of refraction are substantially equal. The match ofrefractive indices associated with that axis, which typically, but notnecessarily, is an axis transverse to the direction of orientation,results in substantially no reflection of light in that plane ofpolarization.

One exemplary embodiment of a polymer fiber that has internalbirefringent interfaces, and that may be used in some embodiments ofpolarizers discussed above, is a multilayer fiber. A multilayer fiber isa fiber that contains multiple layers of different polymer materials, atleast one of which is birefringent. In some exemplary embodiments, themultilayer fiber contains a series of alternating layers of a firstmaterial and a second material, where the first material is opticallyisotropic and the second material is birefringent, having the refractiveindex along one axis about the same as that of the first material andthe refractive index along an orthogonal axis different from that of theisotropic material. Such structures are discussed at greater length in,for example, U.S. Pat. No. 5,882,774, incorporated herein by reference.

A cross-section through one exemplary embodiment of a multilayer fiber400 is schematically illustrated in FIG. 4A. The fiber 400 containsalternating layers of a first material 402 and a second material 404.The first material is birefringent and the second material issubstantially isotropic, so that the interfaces 406 between adjacentlayers are birefringent. In this particular embodiment, the interfaces406 may be substantially planar, and extend along the length of thefiber 400.

The fiber 400 may be surrounded by a cladding layer 408. The claddinglayer 408 may be made of the first material 402, the second material404, the material of the polymer matrix in which the fibers areembedded, or some other material. The cladding 408 may functionallycontribute to the performance of the overall device, or the cladding 408may perform no function. The cladding 408 may functionally improve theoptics of the reflective polarizer, such as by minimizing thedepolarization of light at the interface of the fiber and the matrix.Optionally, the cladding 408 may mechanically enhance the polarizer,such as by providing the desired level of adhesion between the fiber andthe continuous phase material. In some embodiments, the cladding 408 maybe used to provide an antireflection function, for example by providingsome refractive index matching between the fiber 400 and the surroundingpolymer matrix.

The fiber 400 may be formed with different numbers of layers and withdifferent sizes, depending on the desired optical characteristics of thefiber 400. For example, the fiber 400 may be formed with from about tenlayers to hundreds of layers, with an associated range in thickness.There is no limitation on the width of the fiber 400, although preferredvalues of the width may fall in a range from 5 μm to about 5000 μm,although the fiber width may also fall outside this range.

A multilayer fiber 400 may be fabricated by coextruding multiple layersof material into a multilayer film, followed by a subsequent step ofstretching so as to orient the birefringent material and producebirefringent interfaces. Multilayer fibers may be obtained by slicing amultilayer sheet. Some approaches to manufacturing multilayer sheetscontaining birefringent interfaces are described further, for example,in U.S. Pat. Nos. 5,269,995; 5,389,324; and 5,612,820, incorporated byreference.

Some examples of suitable polymer materials that may be used as thebirefringent material include PET, PEN and various copolymers thereof,as discussed above. Some examples of suitable polymer materials that maybe used as the non-birefringent material include the optically isotropicmaterials discussed above.

Other configurations of multilayer fiber may be used. For example,another exemplary embodiment of multilayer fiber 420 may be formed withconcentric layers of alternating first material 422 and second material424, where the first material 422 is birefringent and the secondmaterial 424 is isotropic. In this exemplary embodiment, the fiber 420includes concentric birefringent interfaces 426, between the alternatinglayers 422, 424, that extend along the fiber 420.

The outer layer 428 of the fiber 420 may be formed of one of the firstand second material, the same polymer material as is used in the polymermatrix of the polarizer, or some other material.

The fiber 420 may be formed with any suitable number of layers and layerthicknesses to provide desired optical characteristics, such asreflectivity and wavelength dependence. For example, the fiber 420 maycontain from 10 layers to hundreds of layers. The concentric fiber 420may be formed by coextruding a multilayer form followed by stretching toorient the birefringent material. Any of the materials listed above foruse in the flat multilayer fiber 400 may also be used in the concentricfiber 420.

Multilayer fibers having different types of cross-sections may also beused. One such example is multilayer fiber 440, schematicallyillustrated in cross-section in FIG. 4C. This fiber includes multiplealternating layers of a first material 442 and a second material 444,where the first material 442 is birefringent and the second material 444is optically isotropic. The birefringent interfaces 446 formed betweenthe different layers have flat portions 446 a and curved portions 446 band extend along the fiber 440. The particular cross-sectional shape ofthe different layers is determined primarily by the shape of thefeedblock used to coextrude the fiber 440 and also by any subsequentforming of the fiber 440.

The outer layer 448 may be formed from the first or second materials,the same material as the polymer matrix that the fiber 440 is embeddedin or some other material. The material of the outer layers 408, 428,448 may be selected to provide desired adhesion properties between thepolymer fibers and the surrounding polymer matrix. For example, in someembodiments, the outer layers 408, 428, 448 may be formed of polyesterresin, a silane, or some other agent used for increasing the adhesionbetween the polymer fibers and the polymer matrix. In other embodiments,the outer layers 408, 428, 448 may be made of a material that reducesthe adhesion between the polymer fibers and the surrounding polymermatrix, for example, fluorocarbon materials, silicone materials and thelike. In some embodiments, an outer layer may be used to provide anantireflection function, for example by providing some refractive indexmatching between the polymer fibers and the polymer matrix.

Where the multilayer fiber contains birefringent interfaces having flatportions on the fiber cross-section, the orientation of the flatportions may be controlled within the polarizer to provide a range ofselected effects. For example, in an exemplary embodiment of a polarizer450, schematically illustrated in FIG. 4D, the polarizer 450 includes amatrix 452 embedded with fibers 454 whose birefringent interfaces aregenerally aligned parallel with the surface 456 of the polarizer 450.Where incident light 458 is unpolarized, the polarizer passes light 460in one polarization state and reflects light 462 in the orthogonalpolarization state. In the illustrated embodiment, the transmitted light460 is polarized out of the plane of the figure and the reflected light462 is polarized parallel to the plane of the figure.

In this exemplary embodiment, the reflected light 462 may include asignificant specular component since the birefringent interfaces of thefibers 454 are aligned parallel to each other. The reflected light 462may also include a diffuse component, for example due to surface anddiffractive effects, and also because of misalignments from trueparallelism among the fibers 454. Furthermore, since the birefringentinterfaces are aligned parallel to the surface 456, the polarizer 450behaves somewhat like a mirror for light in the reflected polarizationstate.

Another exemplary embodiment of polarizer 470 is schematicallyillustrated in cross-section in FIG. 4E. In this polarizer 470, thefibers 454 are aligned with the flat portions of the birefringentinterfaces substantially parallel to each other. In this case, however,the flat portions of the birefringent interfaces are not alignedparallel to the surface 456, but are aligned non-parallel to the surface456. Unpolarized incident light 458 results in the transmission of light460 in the transmission polarization state and the reflection of light462 in the reflection polarization state. In this case, when theincident light is normally incident on the surface 456, the reflectedlight 462 may generally be reflected in a direction non-normal to thesurface 456. The reflected light 462 may be said to be directed to theside of the polarizer 470.

Another exemplary embodiment of polarizer 480 is schematicallyillustrated in cross-section in FIG. 4F. In this polarizer 480, thefibers 454 are not aligned with the flat portions of the birefringentinterfaces all parallel, but are oriented with a desired orientationprofile across the polarizer 480. For the purposes of description, it isuseful to define an angle θ, formed between a normal 482 to the flatportions of the birefringent interfaces and a normal 484 to the surface456. The value of θ may vary across the polarizer 480. In theillustrated embodiment, the fibers 454 towards the left side of thepolarizer 480 are oriented so that the value of θ is +θ₀. Thus, thelight 462L reflected from this portion of the polarizer 480 is directedtowards the right. The fibers 454 towards the right side of thepolarizer 480 are oriented so that that the value of θ is −θ₀, and sothe light 462R reflected from this side of the polarizer is directedtowards the left. At the center of the polarizer, the normals 482, 484to the flat portions of the birefringent interfaces and the surface 456are approximately parallel (i.e., θ=0) and so the light 462C reflectedat the center of the polarizer 480 is reflected at approximately theangle of incidence on the surface 456.

It will be appreciated that the manner in which θ varies across thepolarizer 480 may be selected so as to reflect light with apredetermined profile. For example, in the illustrated embodiment, thereflected light may be approximately brought to a focus in front of thepolarizer. In another exemplary embodiment, not illustrated, the flatportions of the birefringent interfaces may be oriented so that thereflected light is brought to a focus to the side of the polarizer,rather than in front of the polarizer.

Another exemplary embodiment of a polarizer 490 is schematicallyillustrated in FIG. 4G. In this polarizer 490, the flat portions of thebirefringent interfaces of different fibers 454 are oriented randomly.As a result, the reflected light 462 is reflected more or lessdiffusely.

It will be appreciated that the relative orientations of the flatportions of the birefringent interfaces may be selected so that thereflected light is more or less specularly reflected or diffuselyreflected, or reflected with some combination of specular and diffusecharacteristics.

In some exemplary embodiments, the fibers 454 maintain a constantorientation, relative to the surface 456, along their length. In otherexemplary embodiments, some or all of the fibers 454 may be twistedalong their length.

Another exemplary embodiment of a polymer fiber that has internalbirefringent interfaces is a composite fiber, which contains multiplescattering fibers infiltrated with a polymer filler. An example of across-section through an exemplary composite fiber is schematicallyillustrated in FIG. 5A. The composite fiber 500 includes multiplescattering fibers 502 with a filler 504 between the scattering fibers502. In some embodiments, at least one of the scattering fibers 502 orthe filler 504 is birefringent. For example, in some exemplaryembodiments, at least some of the scattering fibers 502 may be formed ofa birefringent material and the filler material 504 may benon-birefringent. In other exemplary embodiments, the scattering fibers502 may be non-birefringent while the filler material 504 isbirefringent. In other embodiments, both the scattering fibers 502 andthe filler 504 may be birefringent. In these different variations, eachinterface 508 between the material of a scattering fiber 502 and thefiller material 504 is an interface between a birefringent material andanother material, i.e., is a birefringent interface, and can contributeto the preferential reflection or scattering of light in a selectedpolarization state. In each of these different embodiments, the polymermatrix, in which the composite fiber is embedded, may be opticallyisotropic or birefringent.

In some other embodiments, the fibers 500 may be made from isotropicscattering fibers 502 with an isotropic filler material 504. In such acase, the matrix in which the fibers 500 are embedded is birefringent.

The composite fiber 500 can take on different cross-sectional shapes. InFIG. 5A, the composite fiber 500 has a circular cross-sectional shape.Other exemplary embodiments of composite fibers 510 and 520, shownschematically in FIGS. 5B and 5C respectively, have elliptical andsquare cross-sectional shapes. Other cross-sectional shapes may be used,for example regular and irregular polygonal shapes, or cross-sectionalshapes that combine curved and straight sides. The illustratedembodiments are intended to be exemplary only, and not to be limiting inany way.

A composite fiber may optionally be provided with an outer layer 506.The outer layer 506 may be used, for example, to affect the adhesionbetween the composite fiber and the polymer matrix in which thecomposite fiber is embedded. In some embodiments, the outer layer 506may be formed of a material that increases the adhesion between thecomposite fiber and the polymer matrix, for example a polyester resincoating, a silane coating or other primer used for increasing theadhesion between the polymer matrix and the polymer fibers. In otherembodiments, the outer layers 506 may be made of a material that reducesthe adhesion between the polymer fibers and the surrounding polymermatrix, for example, fluorocarbon materials, silicone materials and thelike. In some embodiments, the outer layer 506 may be used to provide anantireflection function, for example by providing some refractive indexmatching between the fiber 500 and the surrounding polymer matrix.

The positions of the scattering fibers may be random within thecross-section of the composite fiber, for example as schematicallyillustrated in the exemplary embodiments of FIGS. 5A-5C. Othercross-sectional arrangements of the scattering fibers may be used. Forexample, the scattering fibers 502 may be regularly arranged within thecross-section of the composite fiber 530. For example, the exemplaryembodiment of fiber 530 illustrated in FIG. 5D shows the scatteringfibers 502 arranged in a two dimensional array where the separationdistance between adjacent scattering fibers 502 in the y-direction,d_(y), is the same as the separation distance between adjacentscattering fibers 502 in the z-direction, d_(z). In the exemplaryembodiment of fiber 540, illustrated in FIG. 5E, the scattering fibers502 are arranged in a two dimensional array where the separationdistance, d_(y) in the y-direction is different from the separationdistance d_(z) in the z-direction. The scattering fibers 502 in FIGS. 5Dand 5E lie in a rectangular grid pattern, which is understood to includethe square grid pattern of FIG. 5D. The spacing between adjacentscattering fibers 502 may be, for example, in the range 50 nm-500 nm,where the composite fiber 530, 540 is to be used with visible light.

Other regular arrangements of the scattering fibers 502 are possible.For example, in the composite fiber 550, schematically illustrated incross-section in FIG. 5F, the scattering fibers 502 lie in rows alongthe y-direction where adjacent rows are offset from each other in they-direction. In this particular embodiment, the offset between adjacentrows is such that the scattering fibers 502 are arranged in a hexagonalpattern, rather than a square or rectangular pattern. A variation of thearrangement in FIG. 5F is schematically illustrated for composite fiber555 in FIG. 5G, where the separation between nearest neighbor scatteringfibers 502 is greater in the z-direction than in the y-direction.

In other exemplary embodiments, the scattering fibers 502 may form otherpatterns. For example, the scattering fibers may be arranged so as tofill some, but not all, positions in a regular array. Furthermore,spaces or gaps may be introduced between adjacent scattering fibers orgroups of scattering fibers. The size and distribution of such groups orspaces and gaps may be selected to produce particularly desirablespectral characteristics. For example some arrangements of scatteringfibers may act as photonic crystals for light within particularwavelength ranges, which may lead to spectrally selective reflectionand/or transmission. Photonic crystal photonic fibers are discussedfurther in co-owned U.S. patent application Ser. No. 11/068,158, titled“COMPOSITE POLYMER FIBERS”, filed on even date herewith, having attorneydocket no. 60371US002, and incorporated herein by reference.

Additional exemplary embodiments of composite fiber, showing aninexhaustive selection of possible scattering fiber arrangements is nowdescribed.

In the exemplary embodiment of composite fiber 560 schematicallyillustrated in FIG. 5H, some scattering fibers 502 are arrangedregularly in an area around the center of the fiber 560, but the centerportion of the fiber 560 is clear of scattering fibers. In anotherexample of composite fiber 565, schematically illustrated in FIG. 5I,the scattering fibers 502 are arranged in concentric rings 506. The sizeof the scattering fibers 502 and the size of the gap and/or theconcentric rings may be selected for particular optical properties, suchas transmission and/or reflection. In the example illustrated in FIG.5I, the scattering fibers are shown to be located in a ring at positionsset by a hexagonal grid. This is not a necessary condition, and thescattering fibers 502 may be formed in a radially concentric pattern,for example as is schematically illustrated in FIG. 5J.

In some embodiments, the scattering fibers 502 need not all be the samesize. For example, as is shown for the embodiments of composite fiber570 and 575 illustrated in FIGS. 5J and 5K, the composite fiber mayinclude scattering fibers of different cross-sectional sizes. In theseparticular embodiments, the scattering fibers 502 a are relativelylarger in cross-section than the scattering fibers 502 b. The scatteringfibers 502 may fall into groups of at least two different sizes and, infact, may all be different sizes. Furthermore, a scattering fiber 502may be located at the center of the composite fiber, for example asillustrated in FIG. 5I, or there may be no scattering fiber 502 at thecenter of the composite fiber: for example, scattering fibers 502 a arepositioned surrounding, but not at, the center of the composite fiber570 in FIG. 5J. In practice, the dimensions of the scattering fibers 502may fall within a range, rather than being single-valued. In addition,different scattering fibers 502 may be formed of different materials.

As have been discussed above, the composite fiber need not be circularin shape, and may have a non-circular cross-section. In illustration,FIGS. 6A and 6B show non-circular composite fibers 600, 610 that containscattering fibers 502 in square and hexagonal patterns respectively. Thenon-circular fiber may have its scattering fibers 502 positioned atpoints on a regular grid pattern, but not all positions of the gridpattern need be associated with a scattering fiber 502. For example, thenon-circular composite fiber 620 schematically illustrated in FIG. 6Ccontains scattering fibers 502 positioned on a hexagonal grid, but somegaps 612 may be present between fibers. In addition, the pattern formedby the scattering fibers 502 has no axis of symmetry.

Other exemplary embodiments of non-circular composite fibers 630, 640are schematically illustrated in FIGS. 6D and 6E. These exemplarynon-circular composite fibers 630, 640 are square in cross-section andcontain scattering fibers 502 arranged in different exemplary patterns.The scattering fibers 502 in composite fiber 630 are arranged on ahexagonal grid pattern, whereas the scattering fibers 502 in compositefiber 640 are arranged in a square grid pattern. In each case, there aregaps within the arrangement of scattering fibers 502.

The scope of the invention is intended to cover all arrangements ofscattering fibers within the composite fibers. In some exemplaryarrangements, the relative positions of the scattering fibers, the sizeof the scattering fibers and the difference in the refractive indexbetween the scattering fibers and the filler materials may be set toprovide desired spectrally selective properties, for example inreflection and/or transmission, to the composite fiber. Examples of suchspectrally selective properties include, but are not limited to,reflection and transmission. In some embodiments of composite fiber, thecross-sectional locations of the scattering fibers may lead toincoherent scattering of the incident light. In other embodiments, thelocations of the scattering fibers may lead to coherent effects in thescattered light that give rise to photonic crystalline properties. Theaverage density of scattering fibers within the composite fiber maycover a large range, for example about 1% to about 95%, preferably about10% to about 90% and more preferably about 10% to about 50%, althoughthe scattering fiber density may also fall outside these ranges.Composite fibers are discussed in greater detail in co-owned U.S. patentapplication Ser. No. 11/068,158, titled “COMPOSITE POLYMER FIBERS”,filed on even date herewith, having attorney docket no. 60371US002.

The size of the scattering fibers 502 can have a significant effect onscattering. A plot of scattering effectiveness, the normalized, scaledoptical thickness (NSOT), is shown as a function of mean radius of thescattering fiber, in FIG. 7. The NSOT is given by the followingexpression:

NSOT=τ(1−g)/(tf)

where τ is the optical thickness and equals tk, where k is theextinction cross-section per unit volume (the reciprocal of the meanfree path for extinction), t is the thickness of the diffuser, f is thevolume fraction of diffusers and g is the asymmetry parameter. The valueof g is +1 for pure forward-scattering, −1 for pure back-scattering andzero for equally forward and backward scattering. The calculation usedto produce the plot assumed that the vacuum wavelength of the incidentlight was 550 nm.

As can be seen, the scattering effectiveness peaks at a radius of about150 nm, and has a value of about half the maximum over a radius range ofabout 50 nm-1000 nm. The scattering fibers may have any desiredcross-sectional dimension, but the cross-sectional dimension may be inthe range of about 50 nm to about 2000 nm, and more preferably in therange of about 100 nm to about 1000 nm, for light centered at awavelength of about 550 nm. The cross-sectional dimension is thediameter where the scattering fiber has an approximately circularcross-section, and may be taken as the scattering fiber width fornon-circular fiber cross-sections. The size of the scattering fibers maybe different where the composite fiber is being used for applicationswhere the wavelength of the incident light lies outside the visibleregion of the spectrum, for example in the ultraviolet or infraredregions. In general, a preferred range for the cross-sectional dimensionof the scattering fibers is around λ/10 to around 4λ, where λ is thevacuum wavelength of the light. Where the light is present in a range ofwavelengths, the value of λ may taken as the center value of thewavelength range, although the composite fiber may also be provided withscattering fibers having a range of dimensions.

If the scattering fibers are too small, for example less than about onethirtieth of the wavelength of light within the composite fiber, orbelow about 0.012 μm for light at 550 nm in vacuum, and if the densityof scattering fibers is sufficiently high, for example in the range ofabout 60%-80% of the composite fiber volume, then the polarizer maybehave as a medium with an effective refractive index somewhat betweenthe indices of the scattering fiber and the filler along any given axis.In such a case, little light is scattered. When the scattering fiber'scross-sectional size becomes significantly larger than the lightwavelength, for example at least about three times the wavelength ormore, the scattering efficiency becomes very low and iridescence effectscan occur.

The cross-sectional dimensions of the scattering fibers can varydepending on the desired use of the optical material. Thus, for example,the dimensions of the scattering fibers may vary depending on thewavelength of light that is of interest in a particular application,with different dimensions required for scattering or transmittingvisible, ultraviolet, and infrared light. Generally, however, thedimension of the scattering fibers should be approximately greater thanabout one thirtieth of the smallest wavelength of light in thewavelength range of interest, in the material.

At the upper side of the desired dimensional range, the averagedimension of the scattering fibers is preferably equal to or less thantwice the wavelength of light over the wavelength range of interest, inthe material, and preferably less than 0.5 of the desired wavelength.

The density of the scattering fibers within the composite fiber affectsthe amount of scattering that takes place. It may be useful for thecenter-to-center spacing between the scattering fibers to be about λ/10to about 2λ, where λ is the center or average vacuum wavelength of theincident light.

The scattering fibers may be round in cross-section, but need not beround and may have other cross-sectional shapes. In the exemplarycomposite fiber 650, schematically illustrated in cross-section in FIG.6F, the scattering fibers 652 have a square cross-section. Other shapesof cross-section may be used, for example regular and irregularpolygonal shapes, such as triangular, rectangular or hexagonal, orcross-sectional shapes that combine curved and straight sides. Thecross-sectional shape of the scattering fibers may be a result of theshape of the extrusion die, or may be a result of post-processing theoptical element after extrusion. The intention is not to limit theinvention to scattering fibers having those cross-sectional shapes shownin the illustrations.

The use of scattering fibers having non-circular cross-sections isuseful when the center-to-center fiber spacing is non-uniform, since itpermits the scattering fibers to fill a greater fraction of thecross-sectional area of the composite fiber. For example, if thescattering fibers are arranged on a rectangular grid and thecenter-to-center spacing is twice as large in the y-direction as thez-direction, the scattering fibers fill a greater cross-section of thecomposite fiber if the scattering fibers have an ellipticalcross-section that is twice as long in the y-direction than thez-direction than if the scattering fibers were circular.

Some additional exemplary arrangements of scattering fibers havingnon-circular cross-section are schematically illustrated in FIGS. 6G-6I.The non-circular scattering fibers may be arranged with theircross-sectional shapes arranged in random directions. In otherembodiments, the cross-sections of the scattering fibers may be alignedrelative to each other. For example, in FIG. 6G, the composite fiber 660is formed with a filler 504 embedded with scattering fibers 662 havingan elliptical cross-section. In this particular embodiment, thescattering fibers 662 are aligned with the long axes of theircross-sectional ellipses parallel with the y-axis.

The scattering fibers need not be arranged with their cross-sections allin alignment, but different scattering fibers may have differentalignments within the composite fiber. In the exemplary embodiment ofcomposite fiber 670, schematically illustrated in FIG. 6H, thescattering fibers 672 have an elliptical cross-section and some fibers672 a are arranged with their long axes parallel to the z-axis whileother fibers 672 b are arranged with their short axes parallel to thez-axis. Approximately half of the scattering fibers 672 are aligned ineach direction. Also, the populations of the fibers 672 a and 672 b arearranged regularly within the cross-section of the composite fiber 670.It will be appreciated that the populations of the fibers 672 a and 672b may also be arranged irregularly within the cross-section of thecomposite fiber 670.

Other variations on the illustrated embodiments are possible. Forexample, not all scattering fibers need have the same cross-sectionalshape, size or alignment. Furthermore, the scattering fibers may becross-sectionally aligned to form patterns within the composite fiber.One particular example of such a composite fiber 680 is schematicallyillustrated in FIG. 6I. The filler 504 is embedded with scatteringfibers having two different shapes of cross-section, elliptical fibers682 and circular fibers 684. In the illustrated embodiment, theelliptical fibers 682 are aligned so that the short axes of theirelliptical cross-sections are directed towards the closest circularfiber 684. Other patterns of scattering fibers may be used.

Where the scattering fibers have a non-circular cross-section, thescattering fibers may lie untwisted within the composite fiber, so thata face of the scattering fiber is oriented towards one face of thecomposite fiber along the length of the scattering fiber. In otherexemplary embodiments, the scattering fibers may be twisted within thecomposite fiber, so that, at different points along the length of ascattering fiber, a face of the scattering fiber is oriented towardsdifferent faces of the composite fiber.

In many embodiments of composite fiber, the birefringent interfaces maybe curved or may be flat and unaligned. In such embodiments, the lightis reflected at the birefringent interfaces in many different directionsand so the composite fiber may be described as scattering the light.

While the index mismatch is the predominant factor relied upon topromote polarization dependent scattering within composite fibers, thecross-sectional shape of the composite fibers may also have an effect onscattering. For example, when the scattering fiber is elliptical in across-section, the elliptical cross-sectional shape may contribute toasymmetric diffusion in both back scattered light and forward scatteredlight. The effect can either add or detract from the amount ofscattering from the index mismatch.

In some embodiments, the scattering fibers may have a core and shellconstruction, wherein the core and shell are made out of the same ordifferent materials, or wherein the core is hollow. Thus, for example,the scattering fibers may be hollow fibers of uniform or non-uniformcross section. The interior space of the fibers may be empty, or may beoccupied by a suitable medium which may be a solid, liquid, or gas, andmay be organic or inorganic. The refractive index of the medium may bechosen in consideration of the refractive index difference at thebirefringent interfaces so as to achieve a desired degree of reflectionor scattering at the birefringent interface. Suitable isotropic andbirefringent polymer materials have been discussed above.

One method of making a composite fiber is to coextrude multiplescattering fibers using feedblocks designed for manufacturing compositefibers, sometimes also known as “island-in-the-sea” fibers. Such methodsare discussed in greater detail in Handbook of Fiber Science andTechnology: High Technology Fibers Part D, Vol. 3; Lewin and Preston(editors), Marcel Dekker, 1996, ISBN 0-8247-9470-2, incorporated byreference. Other fiber structures and cross-sectional distributions,including those described in this reference, may be used. The compositefibers may be stretched following extrusion, to orient the birefringentmaterial. A more detailed description of methods for coextrudingelements containing scattering fibers is presented in U.S. patentapplication Ser. No. 11/068,159, “COMPOSITE POLYMERIC OPTICAL FILMS WITHCO-CONTINUOUS PHASES”, attorney docket no. 60401US002, filed on evendate herewith and incorporated herein by reference.

EXAMPLE

In an example of coextruding a composite fiber, a feedblock, having onehundred and eighteen laser-machined plates and eleven end-milled plates,was assembled to have two input ports and about 1000 “island” outputports. Within the feedblock, the polymer paths are all of substantiallyequal length. A cross-section through the resulting coextrudantcomposite fiber is shown in the photograph in FIG. 14. The compositefiber comprised a PEN (90%)/PET (10%) copolymer, as the scattering fiber“islands” in a filler “sea” of a PETG copolyester, Eastar™ 6763,supplied by Eastman Chemical Co., Kingsport, Tenn. The extrudedcomposite fiber is about 200 μm in diameter. The composite fiber was notstretched but, with stretching while maintaining geometric shape, couldreach a diameter of around 25 μm, i.e., a reduction in diameter ofaround 87%. At such a stretch, the spacing between the scattering fiberswould be about 500 nm. The cross-sectional dimensions of the scatteringfibers will depend on the ratio of the flow rates of the two differentpolymer materials.

Appropriate optical properties of the polymer fibers can be achieved aspreviously described with birefringent interfaces through the use offiber internal structures including multilayer configurations, bothconcentric and planar, and multiple small scattering fibers within acomposite fiber (“islands-in-the-sea” fiber), or through otherapproaches. Another method for generating the desired internal structurethat contains polymer birefringent interfaces in a fiber is to use twopolymers which are not miscible (and at least one of which isbirefringent) and extrude or cast or form them into a fiber. Uponprocessing, a continuous phase and a dispersed phase are generated. Withsubsequent processing or orientation, the dispersed phase can assumerod-like or layered structures, depending on the internal structure ofthe polymer fiber. Furthermore, the polymer materials may be oriented sothat there is substantial refractive index matching between the twomaterials for one polarization direction and a relatively large indexmismatch for the other polarization. The generation of a dispersed phasein a film matrix is described in greater detail in U.S. Pat. No.6,141,149, included herein by reference.

This type of birefringent polymer fiber may be referred to as adispersed phase fiber. An example of a dispersed phase fiber 800 isschematically illustrated in FIG. 8A, the dispersed phase 802 within thecontinuous phase 804. The end face 806 is a cross-section to show therandom distribution of dispersed phase portions 802 across thecross-section of the fiber 800. The interfaces between the matrix 804and the dispersed phase 802 are birefringent interfaces, and sopolarization sensitive reflection or scattering occurs at theinterfaces.

The dispersed phase may also be formed of liquid crystal droplets,liquid crystal polymers or polymers. The dispersed phase could,alternatively, be comprised of air (microvoids). In any case, theinterfaces between the dispersed and continuous phases within thedispersed phase fiber can induce desired optical properties, includingreflective polarization.

In another approach to forming a birefringent polymer fiber, a fiber maybe formed in a manner similar to a composite fiber, with a first polymerbeing used as the filler, but with second and third polymers being usedfor the scattering fibers. In some embodiments, the second and thirdpolymers are not miscible with each other, and at least one of thesecond and third polymers is birefringent. The second and third polymersmay be mixed and extruded as scattering fibers in a composite fiber.Upon processing, the first polymer forms the filler portion of thecomposite fiber, and the scattering fibers contain both a continuousphase and a dispersed phase, from the second and third polymers,respectively. This type of fiber is referred to as a dispersed phasecomposite fiber. An example of a dispersed phase composite fiber 850 isschematically illustrated in FIG. 8B, showing scattering fibers 852 thatinclude disperse phases 854. The scattering fibers 852 are surrounded bythe filler 856. In other embodiments, the scattering fibers may beformed of a second polymer and a third material, where the thirdmaterial is a liquid crystal material, a liquid crystal polymer or apolymer.

Similarly, the concentric multilayer fiber and non-concentric multilayerfibers may be made of alternating layers with one of the layer typescomprised of a first polymer and the second layer type comprised of amixture of two polymers or materials which are not miscible. Uponprocessing in those cases, alternating layers are produced with somelayers comprising the first polymer and some other layers comprisingboth a dispersed phase and a continuous phase. Preferably, one or bothof the continuous phase and the dispersed phase are birefringent. Withsubsequent processing or orientation, the dispersed phase in the secondtype of layers can assume rod-like or layered structures.

The size requirements for the scattering fibers or birefringent regionsin a layered fiber are similar among all the various embodiments. Thesize of the fiber or thickness of a layer in a multilayer device willneed to be scaled up or down appropriately to achieve the desired sizescale for the systems comprising layers or fibers containing acontinuous and disperse phase, dependent on the desired operatingwavelength or wavelength range.

Another type of polymer fiber that may be used in a polarizer of thepresent invention is now described with reference to FIG. 9. The fiberis formed as a yarn 900. In some embodiments of the yarn 900, the fiberis formed of a number of birefringent polymer fibers 902 twistedtogether, for example by twisting together a number of multilayerfibers, disperse phase fibers, composite fibers, and/or disperse phasecomposite fibers. The yarn 900 may be formed by twisting oriented fiberstogether to form the yarn, or may be formed by twisting isotropic fiberstogether, where the fibers are made of an orientable material, and thenstretching the yarn 900 to orient the orientable material.

The yarn 900 is not restricted to containing only the birefringentpolymer fibers, and may also include other fibers, for example fibers ofother polymeric materials, isotropic or birefringent; natural fibers,such as cotton, silk or hemp; and inorganic fibers such as glass,glass-ceramic, or ceramic fibers.

The yarn 900 may include fiber or lengths of fiber comprised of glass,ceramic, and/or glass-ceramic materials. Glass-ceramic materialsgenerally comprise 95-98 volume percent of very small crystals, with asize generally smaller than 1 micron. Some glass-ceramic materials havea crystal size as small as 50 nm, making them effectively transparent atvisible wavelengths, since the crystal size is so much smaller than thewavelength of visible light. These glass-ceramics can also have verylittle, or no, effective difference between the refractive index of theglassy and crystalline regions, making them visually transparent. Inaddition to the transparency, glass-ceramic materials can have a rupturestrength exceeding that of glass, and are known to have coefficients ofthermal expansion of zero or that are even negative in value.Glass-ceramics of interest have compositions including, but not limitedto, Li₂O—Al₂O₃—SiO₂, CaO—Al₂O₃—SiO₂, Li₂O—MgO—ZnO—Al₂O₃—SiO₂,Al₂O₃—SiO₂, and ZnO—Al₂O₃—ZrO₂—SiO₂, Li₂O—Al₂O₃—SiO₂, andMgO—Al₂O₃—SiO₂.

Some ceramics also have crystal sizes that are sufficiently small thatthey can appear transparent if they are embedded in a matrix polymerwith an index of refraction appropriately matched. The Nextel™ Ceramicfibers, available from 3M Company, St. Paul, Minn., are examples of thistype of material, and are already available as thread, yarn and wovenmats. Suitable ceramic or glass-ceramic materials are described furtherin Chemistry of Glasses, 2^(nd) Edition (A. Paul, Chapman and Hall,1990) and Introduction to Ceramics, 2^(nd) Edition (W. D. Kingery, JohnWiley and Sons, 1976), the relevant portions of both of which areincorporated herein by reference.

The yarn 900 may include lengths of fiber, commonly referred to asstaple fiber, that do not extend over the entire length of the yarn 900.The yarn 900 may be encapsulated within the polymer matrix, with thematrix filling the spaces between the fibers 902 that comprise the yarn900. In other embodiments, the yarn 900 may have a filler between thefibers 902.

Suitable birefringent polymer materials for use in the yarn 900 include,but are not limited to, polymers including PET, PEN, copolymerscontaining terephthalates, naphthalates, or both. In other approaches,the yarn 900 may include birefringent fibers, such as multilayer orcomposite fibers, twisted together.

In general, the birefringent interfaces of the polymer fibers areelongated, extending in a direction along the fibers. In some exemplaryembodiments, the birefringent fibers lie parallel to the x-axis, and sothe diffusely reflected light is scattered mostly into the planeperpendicular to the fibers, the y-z plane, and there is littlescattering in the x-z-plane.

One approach to fabricating a polarizer according to the presentinvention is now discussed with reference to FIGS. 10A-10D. Polymerfibers 1002 are laid on a first polymer layer 1004, as schematicallyillustrates in FIG. 10A. A second polymer layer 1006 may be cast overthe polymer fibers 1002, FIG. 10B. The first polymer layer 1004 and thesecond polymer layer 1006 may be of the same polymer material, or may bedifferent materials. If necessary, the second polymer 1006 layer may beinfiltrated into the fibers 1002 through a variety of methods, forexample, heat and pressure, solvent coating and drying, in-situpolymerization, or a combination thereof.

The fibers 1002 may be laid individually, and may comprise a stack offibers having a height the same as or greater than the width of anindividual polymer fiber, or may be laid as one or more tows. A tow isan arrangement of fibers that are not twisted together. The fibers 1002may be composite fibers, multilayer fibers, fiber yarn, any othersuitable type of fibers, or a combination thereof. In particular, thetow or tows may form a set of fibers that are substantially parallel toeach other. An embodiment of a fiber tow 1106 is schematicallyillustrated in FIG. 11. Cross-members 1108 may be present to providesupport to the polymer fibers 1002 and to keep the polymer fibers 1002at a desired spacing relative to their neighbors. Cross-members 1108need not be present, for example if the polymer fibers 1002 are laidover the first polymer layer 1004 in a continuous process.

The fibers 1002 may also be laid on the first layer 1004 as part of oneor more weaves. A weave 1206 is schematically illustrated in FIG. 12 inwhich the polymer fibers 1002 form the warp and cross-fibers 1208 formthe weft. The cross-fibers 1208 may be made of any suitable fibermaterial, organic or inorganic, and may be, for example, polymer fibers,such as isotropic and/or birefringent polymer fibers, or natural fibers,such as cotton, silk and hemp. In other exemplary embodiments, the crossfibers 1208 may be glass fibers, for example E-glass or S-glass fibers,glass-ceramic fibers or ceramic fibers as discussed above. Therefractive index of the cross-fibers 1208 may be substantially matchedto that of the surrounding polymer matrix so that the cross-fibers havea reduced optical effect on light passing within the polarizer. Inaddition, not all of the warp fibers need be polymer fibers containingbirefringent interfaces. For example, some of the warp fibers may alsobe isotropic fibers, and may be formed of the same type of fiber as thecross-fibers. In some embodiments the warp fibers 1002 may be fibershaving birefringent interfaces that are effective for reflecting orscattering light one polarization of light in one particular wavelengthband. The cross-fibers 1208 may be fibers having birefringent interfaceseffective for reflecting or scattering light one polarization of lightin another particular wavelength band.

The weave may be formed using any suitable weaving process. For example,the weave may be a plain weave, as illustrated, a twill weave, a satinweave, or some other kind of weave. In some exemplary embodiments, thebirefringent fibers 1002 are relatively flat within the weave, forexample as shown schematically in the partial cross-section in FIG. 13A.Note that this figure conforms to the convention that the birefringentfibers 1002 lie substantially parallel to the x-direction. In someexemplary embodiments, the polymer fiber, such as a composite fiber or amultilayered fiber, maintains a single orientation in the weave, withoutbeing twisted.

In other embodiments, the polymer fibers 1002 need not be flat withinthe weave. An exemplary partial cross-section in such a weave 1206 isschematically illustrated in FIG. 13B. It is important to note that theview in this figure is different from that of FIG. 13A. FIG. 13A, showsthe side of the cross-fiber 1208, whereas FIG. 13B shows the side of thepolymer fiber 1002. The coordinate axes conform to the convention usedin earlier figures, so the polymer fiber 1002 lies generally in adirection parallel to the x-axis. Since the polymer fiber 1002 undulateswithin the weave 1206, however, the birefringent interfaces of thepolymer fiber 1002 do not all lie exactly parallel to the x-axis.Accordingly, the light reflected or scattered by the fiber 1002 may bescattered at different angles in the x-z plane. In the illustration,light 1302 is incident on the fiber 1002 in a direction perpendicular tothe x-axis, and a portion of the light 1302 is reflected through anangle, α, with a component in the positive x-direction or negativex-direction, depending on whether the light 1302 is incident on thepolymer fiber 1002 at a “downslope” or “upslope”. Thus, the polymerfiber 1002 may also diffuse the reflected light in the x-z plane. Thedegree of diffuse reflection in the x-z plane depends on the shape ofthe polymer fiber 1002 within the weave: the more that portions of thepolymer fiber 1002 depart from being parallel to the x-direction, thegreater the angular distribution of the light in the x-z plane.

The polymer fibers 1002 may also be provided as a non-woven, as choppedfiber or as a chopped fiber mat.

The polarizer may be formed in a batch process, or in a continuousprocess. In a continuous process, the birefringent fiber 1002,preferably in a tow or weave, is laid onto the first polymer layer 1004and then the second polymer layer 1006 may be continuously cast over thebirefringent fiber 1002. The second layer 1006 may be then be cured orallowed to set.

If desired, additional layers of polymer fiber 1002 may be added, alongwith subsequent layers of polymer material 1008. For example, FIGS. 10Cand 10D show the addition of a set of polymer fibers 1002 over thesecond polymer layer 1006, and the application of a third polymer layer1008.

The first polymer layer 1004 may be a thermoplastic polymer or athermoset-type polymer. The second and subsequent polymer layers 1006and 1008 can also be either thermoplastic or thermoset-type polymers.Thermoplastic polymers can be applied to the previous polymer layer 1004and infiltrated into the fiber 1002 through a variety of methodsincluding heat and pressure, solvent coating and drying, or in-situpolymerization. Thermoset-type polymers can be coated and cured onto thefiber 1002 and previous polymer layers 1004 and 1006 through exposure topressure, heat, actinic radiation, and/or elapsed time.

In an alternative approach to fabricating a polarizer, a polymer film1004 having certain desirable optical, physical, or surface propertiescan be used as a substrate onto which the fibers 1002 are laid. Athermoplastic or thermosetting resin or curable composition can beapplied as the second polymer film 1006 to infiltrate the layer orlayers of fiber 1002, and then a second substrate 1008 can be applied tocreate a sandwich-type structure around the fibers 1002 and the secondpolymer film 1006 of the resin or curable composition. The curable resincan then be cured, hardened, or reacted to form a laminate. In this casethe substrates 1004, 1008 can be made from the same materials as thethermoplastic, thermosetting resin or curable composition, or it can bemade from different materials. A broad range of pressure sensitiveadhesives and hot melt adhesives may be used in place of thethermoplastic or thermosetting resin or curable composition for thesecond layer 1006. In some embodiments, the first and second substrates1004, 1008 may be intimately attached to the thermoplastic orthermosetting resin or curable composition 1006 containing the fibers1002. In other embodiments, the first and second substrates 1004, 1008can be removable.

In another exemplary approach to fabricating a polarizer having morethan a single layer of birefringent fibers, two or more layers of fibersmay be laid on top of a first polymer layer, and then a second layer ofpolymer material is cast over the fibers as the polymer matrix in asingle step.

In another exemplary method of fabricating a polarizer with compositefibers, the filler of the composite fibers may be removed, for exampleby dissolving in a solvent, before the composite fibers are embeddedwithin the polymer matrix. The polymer matrix may then be used as thefiller between the scattering fibers of the composite fiber. This methodmay be particularly useful when the composite fibers are provided in atow or in a weave.

Suitable methods for producing composite fibers include extrudingcomposite fibers with birefringent scattering fibers and a solublefiller. Suitable water soluble fillers include polyvinylpyrrolidinone,cellulose acetate, and polyvinyl alcohol. Suitable polyvinyl alcoholincludes that made from polyvinylacetate that is hydrolyzed to a degreeof about 70 to 95%.

The scattering fibers may be extruded in an array, oriented by heatingthe extruded array and applying suitable tension such that thescattering fibers are stretched to give a stretch ratio that results inthe desired values of refractive index.

Oriented arrays of scattering fibers, forming the composite fibers, maybe formed into yarns. The yarns may optionally also incorporate othertypes of fibers. The yarns are preferably oriented in a single directionby forming a tow of fibers or by weaving the fibers to form a fabric.The soluble polymer filler may be removed by washing the yarns at anystage of manufacture after extrusion.

The washed scattering fibers may be infiltrated with a fluid, preferablya curable resin fluid. Any suitable technique may be used to harden theresin, for example the resin may thermally and/or radiation cured toform the matrix that contains the fibers. In some exemplary embodiments,the resin is cured so that the matrix has flat surfaces. In otherexemplary embodiments, the resin may be cured to have a desiredstructure on one or more surfaces. For example, the resin may be curedwhile it has a surface in contact with the microstructured surface of amicroreplication tool. Examples of suitable microstructured surfacesinclude machined metal surfaces, electroformed replicas, or moldedpolymer films. Examples of suitable microstructures formed on the matrixsurface include linear prismatic structures, non-linear prismaticstructures, Fresnel surfaces, microlenses and the like.

Some embodiments of the invention may be used in, for example, liquidcrystal display (LCD) systems and other polarized display systems. Forexample, a reflective, fiber-based, polarizer of the type discussedabove may be used to polarize light propagating to an LCD panel in thebacklight of an LCD system. Such systems include, but are not limitedto, LCD-TVs and LCD monitors, cell-phone displays and other electronicequipment, such as digital still and video cameras, that displayinformation to a user using an LCD panel.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. An optical body, comprising: a polymer matrix; and a plurality ofpolymer fibers disposed within the polymer matrix and orientedsubstantially parallel to a first axis of the matrix, wherein at leastone of the polymer fibers comprises a first polymer material and asecond polymer material, the first and second polymer materials notmiscible with each other and at least one of the polymer materials beingbirefringent, the at least one fiber having elongated internal multiplebirefringent interfaces for reflecting light, the elongated birefringentinterfaces formed between the first polymer material and the secondpolymer material, wherein a refractive index difference at thebirefringent interfaces for light polarized parallel to the first axisis different from a refractive index difference at the birefringentinterfaces for light polarized parallel to a second axis, the secondaxis orthogonal to the first axis.
 2. An optical body as recited inclaim 1, wherein a plurality of the polymer fibers comprises the firstpolymer material and the second polymer material, the plurality offibers having elongated internal multiple birefringent interfaces forreflecting light, the elongated birefringent interfaces formed betweenthe first polymer material and the second polymer material, wherein arefractive index difference at the birefringent interfaces for lightpolarized parallel to the first axis is different from a refractiveindex difference at the birefringent interfaces for light polarizedparallel to a second axis, the second axis orthogonal to the first axis.3. An optical body as recited in claim 2, wherein the fibers comprises adisperse phase of the second polymer material in a continuous phase ofthe first polymer material.
 4. An optical body as recited in claim 3,wherein the second polymer material has a rod-like structure.
 5. Anoptical body as recited in claim 2, wherein the fibers comprise acontinuous phase of the first polymer material and a continuous phase ofthe second polymer material.
 6. An optical body as recited in claim 5,wherein the first polymer material and the second polymer material areco-continuous.
 7. An optical body as recited in claim 5, furthercomprising a third polymer material in the continuous phase of thesecond polymer material.
 8. An optical body as recited in claim 1,wherein the plurality of fibers are arranged in a one-dimensional arrayin the polymer matrix.
 9. An optical body as recited in claim 8, theplurality of fibers having a regular spacing between adjacent fibers.10. An optical body as recited in claim 8, further comprising a fiberweave within the polymer matrix, the fiber weave comprising theplurality of polymer fibers.
 11. An optical body as recited in claim 10,wherein the plurality of polymer fibers comprise one of the warp and theweft of the fiber weave.
 12. An optical body as recited in claim 1,wherein the polymer matrix has at least one structured surface.
 13. Anoptical body as recited in claim 12, wherein the at least one structuredsurface provides optical power to light transmitted through the body.14. An optical body as recited in claim 12, wherein the at least onestructured surface comprises an array of prismatic structures.